Category: Sea urchins

When I was a boy snorkelling off beaches one of the sights I best remember and one of the potential hazards was the long-spined sea – Diadema setosum. Its quite a beautiful creature with black or brown-banded spines that are extremely long and narrow. They are also hollow and contain a mild venom which -despite being painful – does not pose a serious threat to humans. And if you have ever stepped on one you’ll know the spines are fairly brittle so getting them out is tricky!

They have a black skeleton shaped like a squashed ball that houses all the organs which is called a ‘test’ and essentially that’s the body of the urchin. Get close enough and you will see a bright, orange ring around the urchin’s “anus” (which is on the top of the urchin as you look at the seabed) and some bluish spots surrounding the orange ring. Similar blue spots are arranged in linear fashion along its test.

Adult urchins weigh 35 – 80 grams and measure no more than 7 cm in test diameter and around 4 cm in height. But the spines make them look much bigger!

It is commonly found on coral reefs, but also on sand flats and in seagrass beds. Its range stretches throughout the Indo-Pacific from the Red Sea and then eastward to the Australian coast, then as far north as Japan and as far south as the southern tip of the African east coast. Interestingly it has been introduced into other localities not within its natural range. For example in 2006, two living specimens were found in waters off Turkey.
In Hong Kong it’s very common and lives all across the coast of Eastern Hong Kong as well as the southern side of Lamma and probably anywhere where there is coral or rocks with reef-type algae. Check out Hong Kong Reef Check for more info on its distribution in Hong Kong waters.

During the day they hide in a sheltered crevice and at night they leave it to graze on a variety of algal species in an area about 1 m around their crevice. Very hungry individuals may become carnivorous. They can also bite bits of the rocks and corals producing coral sand as debris.

In Hong Kong they probably spawn in summer when the water temperature reaches at least 25 degrees. But other cues, such as a full moon may affect the spawning, too. They need to live in high densities to reproduce because males and females stimulate the opposite sex to spawn by releasing chemicals into the water and that’s only possible when there lots of them in one place. Females release 10-20 million eggs at each spawning, hoping that at least a few of them will get fertilised by the sperm the males release into the water at the same time. The fertilised eggs develop into larvae that will live and drift for 40-50 days as plankton before settling on the seabed. When the larvae finally settle on a naked rock somewhere, they metamorphose – that is transform into real sea urchins. It’s a bit like caterpillars and butterflies or tadpoles and frogs. Incidentally, almost all marine invertebrates go through such a metamorphosis. Frequently they live below another urchins spines to protect themselves from enemies.